In electronics, a diode is a component that restricts the directional flow of electrical current or charge carriers. In very basic terms, a diode allows electric current to flow in one direction, but essentially blocks flow of current in the opposite direction. Today, the most common diodes are made from inorganic semiconductor materials such as silicon or germanium.
Organic semiconductors are potentially advantageous for low-cost large-area circuits [T. N. Jackson, Nat. Mater. 2005, 4, 581; D. J. Gundlach, Nat. Mater. 2007, 6, 173; T. W. Kelley, P. f. Baude, C. Gerlach, D. E. Ender, D. Muyres, M. A. Haase, D. Vogel, and S. D. Theiss, Chem. Of Mater. 2004, 16, 4413; R. Parashkov, E. Becker, T. Riedel, H.-H. Johannes, and W. Kowalsky, Proc. Of The IEEE 2005, 93, 1321] based on transistors [F. Garnier, R. Hajlaoui, A. Yassar, and P. Shirakawa, Science 1994, 265, 1684; M. Muccini, Nat. Mater. 2006, 5, 605], diodes [L. S. Roman, M. Berggren and O. Inganas, Appl. Phys. Lett. 1999, 75, 3557; K. M. Chen, Y. X. Zhang, G. G. Qin, S. X. Jin, K. Wu, C. Y. Li, Z. N. Gu, and X. H. Zhou, Appl. Phys. Lett, 1996, 69, 3557], and memory elements [L. Ma, S. Pyo, J. Ouyang, Q. Xu, and Y. Yang, Appl. Phys Lett. 2003, 82, 1419]. The first widespread application will likely be flexible displays [L. Zhou, A. Wanga, S. C. Wu, J. Sun, S. Park, and T. N. Jackson, Appl. Phys Lett. 2006, 88, 083502; J. Kido, M. Kimura, and K. nagai, Science 1995, 267, 1332; M. Berggreni, D. Nilsson, and N. D. Robinson, Nat. Mater. 2007, 6, 3; K. Ziemelis, Nature, 1998, 393, 619; T. Lee, Y. Chung, O. Kwon, and J. Park, Adv. Fun. Mater, 2007, 17, 390].[10-14] Another enticing application is the radio frequency identification tag [T. N. Jackson, Nat. Mater. 2005, 4, 581; K. Finkenzeller, RFID Handbook Vol. 1 Ch. 5, 114 Wiley, New York, 2002; E. Cantatore, T. C. T. Geuns, G. H. Gelinck, E. van Veenendaal, A. F. A. Gruijthuijsen, L. Schrijnemakers, S. Drews, and D. M. de Leeuw, IEEE J S. S. Cir, 2007, 42, 84].
Radio-frequency identification (RFID) technology has attracted attention in many domains, such as pharmaceuticals, health, agriculture, transport, logistics, security and more. RFID technology needs to be available cheaply and that can only be possible when the technology to utilize low-cost integrated circuits is available. One solution for RFID is to utilize printable organic materials. These have been considered in recent years for many practical applications such as thin film transistor circuits [C. D. Dimitrakopoulos, P. R. L. Malenfant, Adv. Mater. 14, 99 (2002)], memory [L. P. Mang, S. Pyo, Q. F. Xu, Y. Yang, Appl. Phys. Lett. 82, 1419 (2003)], display [J. Kido, M. Kimura, K. Nagai, Science 267, 1332 (1995).] and diodes [S. Karg, M. Meier, W. Riess, J. Appl. Phys. 82, 1951 (1997); W. Hu, B. Gompf, J. Pflaum, D. Schweitzer, M. Dressel, Appl. Phys. Lett. 84, 4720 (2004)]. However, the performance of a typical organic diode still can not compete with inorganic semiconductor diodes in circuits due to the low carrier mobility and carrier density when organic diodes are operated.
The properties of organic diodes are strongly influenced by the nature and the structure of the interface between the organic film and the metal electrode. Interface states can control carrier injection according to the alignment of the energy bands of the two materials. Only a few investigations of carrier injection and response speed of organic diodes have been reported [P. F. Baude, D. A. Ender, M. A. Haase, T. W. Kelley, D. V. Muyres, S. D. Theiss, Appl. Phys. Lett. 82, 3964, (2003); L. P. Ma, J. Y. Ouyang, Y. Yang, Appl. Phys. Lett. 84, 4786 (2004); S. Steudel, S. D. Vusser, K. Myny, M. Lenes, J. Genoe, P. Heremans, J. Appl. Phys. 99, 114519 (2006); S. Steudel, K. Myny, V. Arkhipov, C. Deibel, S. D. Vussser, J. Genoe, P. Heremans, Nature Material 4, 597 (2005)].
Recently, the Belgian research organization IMEC reported 50 MHz rectifying Schottky diodes made from pentacene [S. Steudel, S. D. Vusser, K. Myny, M. Lenes, J. Genoe, P. Heremans, J. Appl. Phys. 99, 114519 (2006); S. Steudel, K. Myny, V. Arkhipov, C. Deibel, S. D. Vussser, J. Genoe, P. Heremans, Nature Material 4, 597 (2005)]. They used a 300 nm layer of polycrystalline pentacene deposited on gold, with aluminum on top. In order to improve injection-limited current, they added 20 nm of the conducting polymer poly (3,4,ethylenedioxythiophene PEDOT) (PSS) to improve injection from gold. A pentacene diode with PDOT PSS as interface layer had a roughly five times higher maximum current density compared with the pentacene diode without PDOT:PSS. However, for the pentacene diode with PDOT:PSS as an interface layer, high reverse current was observed. In addition, no result was reported relating pentacene film thickness and the organic diode response speed, and all measurements were obtained in inert atmospheres.
A rectifying device can be fabricated as a result of a barrier in a semiconductor junction/semiconductor pn junction or a metal/semiconductor Schottky contact, as has been demonstrated in many previous publications. At Schottky contacts, efficient charge injection can be achieved if the work function of the non-Schottky metal contact is matched to the Fermi level in the semiconductor. Besides metal/organic interfaces, polymer-organic [W. J. E. Beek, M. W. Wienk, and R. A. Janssen, Adv. Mater. 2004, 16, 1009] and polymer/inorganic [W. U. Huynh, J. J. Dittmer, and A. P. Alivisatos, Science 2002, 295, 2425] semiconductor hetero junctions have also been extensively investigated for their carrier injection mechanisms. However organic/inorganic semiconductor junctions have received less attention in optoelectronic devices such as OLEDs. Recently, there have been a few reports on vertical organic Schottky diodes [L. Ma, J. Ouyang, and Y. Yang, Appl. Phys. Lett. 2004, 84, 4786; S. Steudel, K. Myny, V. Arkhipov, C. Deibel, S. De Vusser, J. Genoe, and P. Heremans, Nat. Mater. 2005, 4, 597; S. Steudel, S. De Vusser, K. Myny, M. Lenes, J. Genoe, and P. Heremans, J. Appl. Phys. 2006, 99, 114519; S. De Vusser, S. Steudel, K. Myny, J. Genoe, and P. Heremans, Mater. Res. Soc. Symp. Proc. 2005, 870E, H1.4.1] although their sustained operation at >1 MHz in ambient environment necessitates encapsulation. The Schottky barriers were formed from highly purified pentacene and aluminum top contacts.
For high speed and high current density diodes, the interface resistance should be very low at forward bias. Two other desirable properties of the device are low conductance in reverse bias and a high breakdown voltage. In the case of a p-n junction diode, electrons and holes moving towards the junction at forward bias recombine.
Also, circuits that require an AC signal to be converted to DC current require rectification. Organic semiconductors have been considered as the basis for such circuits, however, the time constant for organic rectifiers is generally too slow to operate on high-frequency AC signals such as in radiofrequency identification (RFID) tags.
It thus would be desirable to provide organic diodes having polymer and/or inorganic layers in combination with the organic layer and each other, and methods related thereto, especially for high speed operation and for operation in the ambient atmosphere.